This invention pertains generally to improved metal wood type golf club heads and more particularly to a golf club head having an improved crown configuration incorporating high specific-strength materials. A recent trend in golf club head design has been to increase the size of such heads to generate increased performance and create more “forgiving” golf clubs. Although this can be said to be true for golf clubs in general, it may be observed that wood type club heads in particular have increased in size dramatically over the past few years. This has presented a number of challenges to designers of modern “metal wood” golf clubs.
Traditional wood type golf club heads generally comprise four primary surfaces that form a solid with predominantly convex outer surfaces. These four primary surfaces are referred to as the striking face (front surface), crown (top surface), skirt (side surface), and sole (bottom surface). In the case of modern metal woods, these surfaces form the exterior of thin metallic walls that are joined or integrally formed to create a thin-walled solid structure. A hosel is typically attached to at least one of the primary surfaces, and serves as a coupling member for attachment of a shaft to the club head. Such metal woods have nominal mass properties including a target mass, a center of gravity, and moments of inertia about a set of axes originating from a reference location (typically the center of gravity, or a point along the hosel axis).
The target mass refers to the ideal total mass for a finished club head, and must be differentiated from a minimum structural mass of a club head. Each club head must have a finished mass that yields a minimum desired swingweight value when assembled to a shaft fitted with a grip. The target mass will depend on the expected maximum length of shaft that may be assembled to the head, and taking into consideration the selection of grips that may be fitted thereto. The swingweight value may then be increased throughout a desired range of values for that shaft length, preferably by adding minor amounts of ballast. For shafts of lesser lengths, the minimum swingweight, and subsequently larger swingweights, may also be achieved by adding more ballast. Therefore the target mass of the head is dictated by the club type, shaft materials and maximum length, as well as the selection of grips which may be fitted thereto.
The minimum structural mass of a club head refers to the minimum mass of all structural components required to produce a club head having a desired shape and geometry that can withstand the loads experienced during normal use. If the minimum structural mass achieved for a given design is less than the target mass, the difference is known as discretionary mass. This amount of discretionary mass may be strategically positioned throughout the club head to fine tune its performance characteristics. Parameters such as center of gravity location, principal axes and the magnitudes of the moments of inertia about them, may all be manipulated through strategic placement of discretionary mass. Thus, it is highly desirable for a club head design to achieve the absolute minimum structural mass to maximize the amount of discretionary mass available to the designer. This amount of discretionary mass available to the designer is also known as the weight budget.
It is known that a low and deep center of gravity generally provides beneficial launch conditions at the moment of impact between a golf club head and ball. Specifically, the combination of a high launch angle and a low ball spinning speed provides increased carry and therefore greater overall distance. Displacing the center of gravity lower in the head (closer to the sole) yields a higher launch angle to the ball at impact, accompanied by increased back spin. Positioning the center of gravity deeper in the club head (farther rearward from the face) will reduce the amount of back spin imparted to the ball at impact. Therefore, for optimum launch conditions of a metal wood, a low and deep club head center of gravity is sought.
A recent trend in metal wood design has been to increase head size in an effort to maximize moments of inertia, thereby minimizing distance loss when a ball is struck other than in the sweet spot of the striking face. However, increased head sizes have generated metal woods with commensurately larger and taller striking faces, which in turn increases the vertical distance between the crown and sole walls. Skirt walls have become correspondingly taller to bridge the larger distances between crown and sole. Therefore, at the minimum structural mass, center of gravity heights have increased in modern club heads.
Further, since the striking face must withstand the greatest loads compared to a remainder of the club head under normal use, it is generally the thickest wall of a metal wood head, and therefore the heaviest. Thus, increases in striking face size have also displaced center of gravity positions farther forward within modern metal wood heads at their minimum structural mass.
Still further, increasing the overall size of modern metal wood club heads has been accompanied by an increase in the volume of material required to form the head, therefore increasing the minimum structural mass, whereas target masses have remained constant. Increasing head volume while maintaining traditional head shapes has therefore resulted in decreased weight budget and a correspondingly reduced ability to improve the mass properties of modern metal wood club heads.
Recent attempts to mitigate increased structural mass have included the advancement of thin-walled casting techniques for metal wood head portions such as the crown, sole, or skirt that may previously have had thicknesses that were greater than necessary for the structural loads placed on them during use. The result has been the achievement of the thinnest possible casting thicknesses for such portions with significant gains in weight budget and therefore the ability to better define the mass properties of metal wood heads. However, it has been demonstrated that there is room for further improvement upon these results, and that it is possible to produce metal wood heads with still more superior performance.
Accordingly, club head manufacturers have advanced club performance by fabricating select head portions from materials having a specific strength (ultimate tensile strength divided by specific gravity) that is greater than conventional head materials such as steel or titanium, while fabricating the rest of the head using conventional metal wood techniques and materials. These types of club heads are generally expensive to manufacture. The head portions are typically attached using various techniques, for example bonding. They can experience reduced durability, and produce a less satisfying sound at impact than a hollow metal wood of advanced thin-wall construction. The sound produced by any golf club at impact has a great deal of influence on a golfer's perception of the quality and performance of the club as a whole, and golfers are particularly demanding of a quality sound produced at impact by metal wood clubs.
Alternative attempts to achieve a minimum structural mass and hence increased weight budget over conventional metal wood head configurations have included the use of composite materials to form the head, e.g. carbon fiber reinforced epoxy or carbon fiber reinforced polymer, in place of traditional materials such as aluminum, steel, and titanium. A primary benefit of using composite materials to construct a head is their improved strength to weight ratios in comparison to traditional materials, permitting a reduction in the head's minimum structural mass, thereby increasing the weight budget available for strategic placement. However, such heads have suffered from durability, performance, and manufacturing issues associated with composite materials. These include higher labor costs in manufacture, undesirable acoustic properties, shearing and separation of composite plies used to form the striking surface of the club head, and comparatively low coefficients of restitution.
In such heads made from composite materials, the areas subject to greatest wear, e.g. the face and sole, have been provided with a metal plate in one or both regions in an attempt at reinforcing those regions. Integrated metal face and hosel constructions have also been attempted with the remainder being formed of composite material, and in several instances such constructions have also included a metal skirt portion. These hybrid constructions have remedied many of the durability issues associated with heads formed entirely of composites while retaining some of the weight budget increase afforded by replacing metal components with a composite material. Furthermore, when a metal is used for the striking face, coefficients of restitution generally similar to those of wood type heads having all-metal construction have been achieved. However, such hybrid constructions are still bound by the inherent disadvantages of a traditional metal wood head shape, including the substantial mass of the crown and skirt portions being concentrated high within the head.
Still other attempts at improving club performance have included the elimination of certain portions of the club head as a whole, most notably the crown, in an attempt to eliminate the contribution of that component's mass from the overall head weight and thereby lower the center of gravity. Such club heads require a great deal of reinforcement in other areas of the head to compensate for the reduced structural integrity due to an open section, which virtually eliminates the possibility of achieving an increased weight budget. Further, such heads have also produced a displeasing sound at impact.
Additionally, club heads which are combinations of the above themes have been manufactured. Such combinations have included club heads where a portion, such as the crown, has been eliminated and certain components, for example the face, have been fabricated from higher specific strength materials. Such variations have yielded disadvantages consistent with the designs mentioned above.
Hence, there exists a need in the art of golf club design for improved metal wood head configurations that provide an improved center of gravity location at the minimum structural mass, and an increased weight budget. In addition, there exists a further need for an additional improvement including use of hybrid material construction, thereby advancing the performance standard of club heads of the metal wood variety to a level not previously attained in the industry.